On the use of LOPA and risk graphs for SIL determination

• An analysis and comparison of advantages and drawbacks or LOPA versus Risk Graph is made.
• It was confirmed that LOPA is a more rigorous and comprehensive method than Risk Graphs.
• Risk Graphs is found useful for screening a large number of safety functions in a phased approach.
• Risk Graphs require additional design and calibration, and to be complemented with a documentation framework.
• Risk graph are usually assumed to be more conservative than LOPA, but this has to be ensured during calibration.
• None of the two methods, LOPA or Risk Graphs, are designed to handle Common Cause Failure.

Safety Integrity Level (SIL), as defined in IEC 61511, is a widely used safety performance measure for safety instrumented functions. The standard IEC 61511 suggests several methods for SIL determination, ranging from fully quantitative methods to fully qualitative methods. The large number of safety functions to evaluate during plant design and the need to integrate multidisciplinary design and operation knowledge to achieve effective risk reduction has made necessary the use of multi-disciplinary-team workshop approaches.Two widely used methods in the Oil & Gas industry for SIL determination are Layer of Protection Analysis (LOPA) and Risk Graphs. Each of these methods has their own advantages and disadvantages. LOPA allows the required risk reduction to be incorporated into the SIL values with higher precision. This enables a more detailed consideration of the available protection layers and leaves an objective traceable record of the decision-making process.In contrast, the simplicity of Risk Graphs makes them convenient for screening a large number of safety functions. This can make Risk Graphs useful as a first screening pass prior to using LOPA. However, Risk Graphs are still widely used as a stand-alone method.This paper seeks to explore the differences between LOPA and Risks graphs and to investigate whether the Risk Graphs method can provide the same level of SIL determination rigor as LOPA. The paper aims to determine if the simplicity of Risk Graphs can make that method more efficient for cases when the number of safety functions to evaluate is considerable.